1. Field of the Invention
This invention relates to an aluminum alloy impeller and a manufacturing method of the same. For example, the present invention relates to a centrifugal compressor impeller utilized as an engine supercharger or impellers of other types which are suitably used for a high-speed rotation requiring light weight and heat resistance, for instance for a rotor and a blade of a turbo molecular pump or a scroll of a scroll compressor.
2. Description of the Related Art
FIG. 1 is a vertical section view illustrating the structure of a conventional centrifugal compressor impeller.
Referring to FIG. 1, there is shown a conventional centrifugal impeller 1. In the case of an atmosphere suction single stage type product in which a compression ratio is relatively low and a temperature in a highest part (compressed air outlet part) increases only up to about 160.degree., the impeller 1 is made of a heat resistant aluminum alloy which is produced by normal dissolving/forging. In the case of a high performance product in which a compression ratio is high or a gas temperature in a suction side is high and a temperature in a highest part (compressed air outlet part) exceeds 200.degree. C., the impeller 1 is made of cast steel or a titanium alloy.
Among the conventional compressors described above, an atmosphere suction single stage type centrifugal compressor in which a compression ratio is relatively low (the compression ratio is up to 3.5) has frequently been used as a compressor for a large diesel engine. For this centrifugal compressor, a higher compression ratio has been requested with the attainment of higher performance in an engine side.
In particular, a centrifugal compressor having a compression ratio of 4.0 or higher has been requested. In this case, an air temperature even reaches 200.degree. C. or higher in the exit of the impeller.
Generally, in such an atmosphere suction single stage type centrifugal compressor, a heat resistant aluminum alloy produced by dissolving/forging is selected as a material for the impeller. In order to attain a sufficient heat resistant strength in particular, a JIS A 2618 alloy of "Al--Cu--Mg" which has highest heat resistance at present is often used.
However, a heat resistant aluminum alloy which has been used hitherto is a type for securing a strength typically by executing an age heat treatment (190.degree. C..times.about 15 hours). Thus, there was a problem of a reduction in a strength when the alloy was heated for a long time in a temperature region exceeding the above-noted temperature.
Accordingly, it has been difficult to secure a high pressure ratio in the case of the impeller which is made of an aluminum alloy. There is now a tendency to shift to a method for using cast steel or a titanium alloy as a material for an impeller.
However, if an impeller is manufactured by using cast steel or a titanium alloy as a material, since the impeller itself has a very complex form, compared with the impeller made of an aluminum alloy, manufacturing costs are much higher.
Compared with the conventional aluminum alloy, a material density is higher for cast steel or a titanium alloy. Accordingly, the weight of the impeller increases, its responsiveness as a centrifugal compressor deteriorates and imbalance during decentering is larger.
Furthermore, because of a large overhung mass, the stability of a rotor is lost and handling is difficult during disassembling.
In recent years, arts for producing a rapid solidification Al--Fe aluminum alloy or an Al--Fe--Mn aluminum alloy by a rapid solidification process has been disclosed (see Japanese Patent Publication No. 63-9576, Japanese Patent Publication No. 63-10221 and Japanese Patent Provisional Publication No. 62-124242). This process is designed to improve the characteristic of an aluminum alloy by rapidly solidifying the aluminum alloy from its molten state and sintering and solidifying obtained powder (heating and pressuring) so as to produce a fine and uniform structure.
However, with these arts, the ductility of an obtained rapid solidification aluminum alloy is extremely low not only in a room temperature but also even in a high temperature, which makes it impossible to perform hot free forging. Thus, it was impossible to obtain a material which could be suitably used as a reliable member for a large high-speed rotary impeller, and so on, to which a large force was applied.
Among the conventional aluminum alloy producing processes, a powder metallurgy process (referred to as a PM process, hereinafter) for producing rapid solidification powder having a specified composition by a gas atomizing process and obtaining a billet by sintering and solidifying this powder (heating and pressurizing) has mainly been used. However, this process needed many steps such as powder classification, can sealing, degassing and hot extrusion processing and was technically complex, and it was difficult to set many conditions. Accordingly, producing costs inevitably increased.
Furthermore, as a sintered body, its strength, rigidity and other characteristics were improved. However, since its ductility was low not only in a room temperature but also even in a high temperature, secondary processing after sintering was difficult and extrusion processing and free forging of a large billet were practically impossible.
Consequently, the application of the above-noted PM process was limited only to such small members as a piston for a small engine, a connecting rod and a member for an electric apparatus.